A method for controlling a vapour compression system during gas bypass valve malfunction

ABSTRACT

A method for controlling a vapour compression system (1) is disclosed. Malfunctioning of a gas bypass valve (8) is registered. An actual opening degree of the gas bypass valve (8) is derived, and a target opening degree of the gas bypass valve (8) is derived, based on one or more control parameters of the vapour compression system (1). The actual opening degree is compared to the target opening degree, and the vapour compression system (1) is controlled based on the comparison, and in order to match a mass flow of gaseous refrigerant through the gas bypass valve (8) to the actual opening degree of the gas bypass valve (8).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Stage application of International PatentApplication No. PCT/EP2017/079359, filed on Nov. 15, 2017, which claimspriority to Danish Patent Application No. PA201600722, filed on Nov. 22,2016, each of which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

The present invention relates to a method for controlling a vapourcompression system, such as a refrigeration system, a heat pump or anair condition system, in which the vapour compression system is allowedto keep operating if a gas bypass valve, interconnecting a gaseousoutlet of a receiver and an inlet of a compressor, is malfunctioning.

BACKGROUND

Vapour compression systems, such as refrigeration systems, heat pumps orair condition systems, are normally controlled in order to provide arequired cooling or heating capacity in an as energy efficient manner aspossible. Some vapour compression systems are provided with a receiverarranged in the refrigerant path downstream relative to an outlet of aheat rejecting heat exchanger and upstream relative to an expansiondevice arranged to supply refrigerant to an evaporator. In the receiver,refrigerant is separated into liquid refrigerant and gaseousrefrigerant. The liquid part of the refrigerant is supplied to theexpansion device, via a liquid outlet of the receiver. At least some ofthe gaseous part of the refrigerant may be supplied directly to an inletof a compressor, via a gaseous outlet of the receiver and a gas bypassvalve. The mass flow of gaseous refrigerant from the gaseous outlet ofthe receiver towards the inlet of the compressor can be controlled bycontrolling an opening degree of the gas bypass valve.

In the case that the gas bypass valve malfunctions, it may be stuck in acertain position defining a certain opening degree of the valve. In thiscase it is no longer possible to control the mass flow of gaseousrefrigerant from the gaseous outlet of the receiver towards the inlet ofthe compressor. During normal operation of a vapour compression system,the opening degree of the gas bypass valve may vary to a great extent,in order to meet system requirements, adapt to ambient conditions, andallow the vapour compression system to provide a required cooling orheating capacity. Therefore, the opening degree defined by themalfunctioning gas bypass valve will most likely not match a desiredopening degree most of the time. This may lead to situations in whichthe vapour compression system becomes unstable or is not capable ofproviding a required cooling or heating capacity, if the vapourcompression system continues operating without controlling the openingdegree of the gas bypass valve. Accordingly, it may be necessary to shutdown the vapour compression system and to request immediate service onthe system. This is very undesirable and may be very costly.

SUMMARY

It is an object of embodiments of the invention to provide a method forcontrolling a vapour compression system, in which the vapour compressionsystem is allowed to continue operating in the case of gas bypass valvemalfunction.

It is a further object of embodiments of the invention to provide amethod for controlling a vapour compression system, in which the numberof shut down events are reduced as compared to prior art methods.

The invention provides a method for controlling a vapour compressionsystem, the vapour compression system comprising at least onecompressor, a heat rejecting heat exchanger, a high pressure expansiondevice, a receiver, an evaporator expansion device, an evaporator and agas bypass valve, arranged in a refrigerant path, the method comprisingthe steps of:

-   -   registering that the gas bypass valve is malfunctioning,    -   deriving an actual opening degree of the gas bypass valve,    -   deriving a target opening degree of the gas bypass valve, based        on one or more control parameters of the vapour compression        system,    -   comparing the actual opening degree of the gas bypass valve to        the target opening degree of the gas bypass valve, and    -   controlling the vapour compression system based on the        comparison, and in order to match a mass flow of gaseous        refrigerant through the gas bypass valve to the actual opening        degree of the gas bypass valve.

The method according to the invention is a method for controlling avapour compression system. In the present context the term ‘vapourcompression system’ should be interpreted to mean any system in which aflow of fluid medium, such as refrigerant, circulates and isalternatingly compressed and expanded, thereby providing eitherrefrigeration or heating of a volume. Thus, the vapour compressionsystem could, e.g., be a refrigeration system, an air condition systemor a heat pump.

The vapour compression system comprises at least one compressor, a heatrejecting heat exchanger, a high pressure expansion device, a receiver,an evaporator expansion device, an evaporator and a gas bypass valve,arranged in a refrigerant path. Refrigerant flowing in the refrigerantpath is compressed by the compressor(s) before being supplied to theheat rejecting heat exchanger. In the heat rejecting heat exchanger,heat exchange takes place between the refrigerant and the ambient or asecondary fluid flow across the heat rejecting heat exchanger, in such amanner that heat is rejected from the refrigerant flowing through theheat rejecting heat exchanger. The heat rejecting heat exchanger may bein the form of a condenser, in which case at least part of therefrigerant passing through the heat rejecting heat exchanger iscondensed, and the refrigerant leaving the heat rejecting heat exchangeris, in this case, at least partly in a liquid state. As an alternative,the heat rejecting heat exchanger may be in the form of a gas cooler, inwhich case the refrigerant passing through the heat rejecting heatexchanger is cooled, but remains in a gaseous state.

The refrigerant leaving the heat rejecting heat exchanger passes throughthe high pressure expansion device before being supplied to thereceiver. In the high pressure expansion device, the refrigerantundergoes expansion, and the refrigerant received in the receiver istherefore a mixture of liquid and gaseous refrigerant. The high pressureexpansion device may, e.g., be in the form of a high pressure valve. Asan alternative, the high pressure expansion device may be in the form ofan ejector. As another alternative, the high pressure expansion devicemay include at least one high pressure valve and at least one ejectorarranged in parallel.

In the receiver the refrigerant is separated into a liquid part and agaseous part. The liquid part of the refrigerant is supplied to theevaporator expansion device, via a liquid outlet of the receiver. Theevaporator expansion device controls a supply of refrigerant to theevaporator, and the refrigerant undergoes expansion when passing throughthe evaporator expansion device. Accordingly, the refrigerant beingsupplied to the evaporator is a mixture of liquid and gaseousrefrigerant. The evaporator expansion device may, e.g., be in the formof an expansion valve.

In the evaporator the liquid part of the refrigerant is evaporated,while heat exchange takes place between the refrigerant and the ambientor a secondary fluid flow across the evaporator, in such a manner thatheat is absorbed by the refrigerant passing through the evaporator.Finally, the refrigerant is supplied to an inlet of the compressor(s).

At least part of the gaseous part of the refrigerant in the receiver maybe supplied directly to the inlet of the compressor(s), via a gaseousoutlet of the receiver and the gas bypass valve. Accordingly, the gasbypass valve controls the supply of gaseous refrigerant from thereceiver to the compressor(s).

According to the method of the invention, it is initially registeredthat the gas bypass valve is malfunctioning. As described above, whenthe gas bypass valve malfunctions, it is no longer possible to controlthe supply of gaseous refrigerant from the receiver to thecompressor(s).

Next, an actual opening degree of the gas bypass valve is derived. Theactual opening degree may, e.g., be an opening degree at which the gasbypass valve is stuck. The actual opening degree of the gas bypass valvemay be retrieved from a controller used for controlling the gas bypassvalve. However, when the gas bypass valve is malfunctioning, the openingdegree which is registered by the controller may differ from the actualopening degree, e.g. because the gas bypass valve is stuck, and hastherefore not adjusted the opening degree to a position specified by thecontroller. Accordingly, it will often be necessary to derive the actualopening degree in another manner. This will be described in furtherdetail below.

Furthermore, a target opening degree of the gas bypass valve is derived,based on one or more control parameters of the vapour compressionsystem. Thus, the target opening degree of the gas bypass valve is anopening degree which provides an optimal mass flow of gaseousrefrigerant from the gaseous outlet of the receiver towards the inlet ofthe compressor(s), under the prevailing operating conditions.Accordingly, if the gas bypass valve had not been malfunctioning, theopening degree of the gas bypass valve would have been set to the targetopening degree.

Next, the actual opening degree of the gas bypass valve is compared tothe target opening degree of the gas bypass valve. Thereby it isrevealed how much the actual opening degree of the gas bypass valve isoff relative to the target opening degree.

Finally, the vapour compression system is controlled based on thecomparison, and in order to match a mass flow of gaseous refrigerantthrough the gas bypass valve to the actual opening degree of the gasbypass valve.

Thus, under normal operation of the vapour compression system, where thegas bypass valve is fully operational, the opening degree of the gasbypass valve is controlled in order to provide a mass flow of gaseousrefrigerant from the gaseous outlet of the receiver towards the inlet ofthe compressor(s) which matches the given operating conditions. However,when the gas bypass valve is malfunctioning, the vapour compressionsystem is instead operated in such a manner that the operatingconditions are adjusted to match the actual opening degree of the gasbypass valve, in the sense that the mass flow of gaseous refrigerantthrough the gas bypass valve is adjusted to match the actual openingdegree of the gas bypass valve. Thereby the disadvantages related to amismatch between operating conditions and a gas bypass valve being stuckat a fixed opening degree are avoided, and operation of the vapourcompression system can be continued, at least for a limited period oftime, until service personnel arrives to repair or replace themalfunctioning gas bypass valve. Thereby shut down of the vapourcompression system and possible damage to goods stored in display casesof the vapour compression system is avoided.

The step of controlling the vapour compression system based on thecomparison may comprise the steps of:

-   -   in the case that the comparison reveals that the actual opening        degree of the gas bypass valve is larger than the target opening        degree of the gas bypass valve, controlling the vapour        compression system to increase the mass flow of gaseous        refrigerant through the gas bypass valve, and    -   in the case that the comparison reveals that the actual opening        degree of the gas bypass valve is smaller than the target        opening degree of the gas bypass valve, controlling the vapour        compression system to decrease the mass flow of gaseous        refrigerant through the gas bypass valve.

According to this embodiment, in the case that it is revealed that theactual opening degree of the gas bypass valve is larger than the targetopening degree of the gas bypass valve, then the gas bypass valve hasmore capacity than requested by the current operating conditions.Accordingly, in this case it is desirable to adjust the operatingconditions in such a manner that the mass flow of gaseous refrigerantthrough the gas bypass valve is increased. Thereby the actual flow ofgaseous refrigerant through the gas bypass valve is adjusted to matchthe higher capacity defined by the actual opening degree of the gasbypass valve.

Similarly, in the case that it is revealed that the actual openingdegree of the gas bypass valve is smaller than the target opening degreeof the gas bypass valve, then the gas bypass valve has less capacitythan requested by the current operating conditions. Accordingly, in thiscase it is desirable to adjust the operating conditions in such a mannerthat the mass flow of gaseous refrigerant through the gas bypass valveis decreased. Thereby the actual flow of gaseous refrigerant through thegas bypass valve is adjusted to match the lower capacity defined by theactual opening degree of the gas bypass valve.

The step of increasing the mass flow of gaseous refrigerant through thegas bypass valve may comprise decreasing a pressure of refrigerantleaving the heat rejecting heat exchanger and/or increasing atemperature of refrigerant leaving the heat rejecting heat exchanger.

When a pressure of refrigerant leaving the heat rejecting heat exchangeris decreased, the gas to liquid ratio of refrigerant in the receiver isincreased. Thereby the mass flow of gaseous refrigerant through the gasbypass valve is increased.

The pressure of refrigerant leaving the heat rejecting heat exchangercould, e.g., be decreased by increasing an opening degree of the highpressure expansion device.

When a temperature of refrigerant leaving the heat rejecting heatexchanger is increased, the gas to liquid ratio of refrigerant in thereceiver is increased. Thereby the mass flow of gaseous refrigerantthrough the gas bypass valve is increased.

The temperature of refrigerant leaving the heat rejecting heat exchangercould, e.g., be decreased by increasing a fan speed of a fan controllinga secondary fluid flow across the heat rejecting heat exchanger.

Similarly, the step of decreasing the mass flow of gaseous refrigerantthrough the gas bypass valve may comprise increasing a pressure ofrefrigerant leaving the heat rejecting heat exchanger and/or decreasinga temperature of refrigerant leaving the heat rejecting heat exchanger.

When a pressure of refrigerant leaving the heat rejecting heat exchangeris increased, the gas to liquid ratio of refrigerant in the receiver isdecreased. Thereby the mass flow of gaseous refrigerant through the gasbypass valve is decreased.

The pressure of refrigerant leaving the heat rejecting heat exchangercould, e.g., be increased by decreasing an opening degree of the highpressure expansion device.

When a temperature of refrigerant leaving the heat rejecting heatexchanger is decreased, the gas to liquid ratio of refrigerant in thereceiver is decreased. Thereby the mass flow of gaseous refrigerantthrough the gas bypass valve is decreased.

The temperature of refrigerant leaving the heat rejecting heat exchangercould, e.g., be increased by decreasing a fan speed of a fan controllinga secondary fluid flow across the heat rejecting heat exchanger.

The step of controlling the vapour compression system based on thecomparison step may comprise adjusting an opening degree of the highpressure expansion device, adjusting a secondary fluid flow across theheat rejecting heat exchanger and/or adjusting a compressor capacity ofthe compressor(s).

As described above, adjusting an opening degree of the high pressureexpansion device will result in a change in the mass flow of gaseousrefrigerant through the gas bypass valve, since an increase in theopening degree of the high pressure expansion device results in adecrease in the pressure of refrigerant leaving the heat rejecting heatexchanger, and a decrease in the opening degree of the high pressureexpansion device results in an increase in the pressure of refrigerantleaving the heat rejecting heat exchanger.

Furthermore, as described above, adjusting a secondary fluid flow acrossthe heat rejecting heat exchanger, e.g. by adjusting a fan speed of afan controlling the secondary fluid flow, will result in a change in themass flow of refrigerant through the gas bypass valve. Moreparticularly, an increase in the secondary fluid flow across the heatrejecting heat exchanger results in a decrease in the temperature ofrefrigerant leaving the heat rejecting heat exchanger, and a decrease inthe secondary fluid flow across the heat rejecting heat exchangerresults in an increase in the temperature of refrigerant leaving theheat rejecting heat exchanger.

Finally, adjusting a compressor capacity of the compressor(s) results ina change in the pressure in the suction line, and thereby in a change inthe pressure difference across the gas bypass valve. Thus, increasingthe compressor capacity results in a decrease in the pressure in thesuction line, and thereby in an increase in the pressure differenceacross the gas bypass valve. This will, in turn, result in an increasein the mass flow of gaseous refrigerant through the gas bypass valve.Similarly, decreasing the compressor capacity results in an increase inthe pressure in the suction line, and thereby in a decrease in thepressure difference across the gas bypass valve. This will, in turn,result in a decrease in the mass flow of gaseous refrigerant through thegas bypass valve.

The vapour compression system may be of a kind which comprises one ormore main compressors and one or more receiver compressors. In this casethe main compressor(s) is/are connected to the outlet of the evaporatoras well to the gaseous outlet of the receiver, via the gas bypass valve,and the receiver compressor(s) is/are connected directly to the gaseousoutlet of the receiver. In this case adjusting the compressor capacitycould include adjusting the compressor capacity of the receivercompressor(s). Increasing the compressor capacity of the receivercompressor(s) results in an increased mass flow of gaseous refrigerantfrom the receiver to the receiver compressor(s). Thereby the pressureprevailing inside the receiver is decreased, thereby decreasing thepressure difference across the gas bypass valve, and decreasing the massflow of gaseous refrigerant through the gas bypass valve. Similarly,decreasing the compressor capacity of the receiver compressor(s) resultsin a decreased mass flow of gaseous refrigerant from the receiver to thereceiver compressor(s). Thereby the pressure prevailing inside thereceiver is increased, thereby increasing the pressure difference acrossthe gas bypass valve, and increasing the mass flow of gaseousrefrigerant through the gas bypass valve.

The step of deriving an actual opening degree of the gas bypass valvemay comprise the steps of:

-   -   obtaining one or more refrigerant pressure values and one or        more refrigerant temperature values at selected positions along        the refrigerant path,    -   estimating a mass flow of gaseous refrigerant through the gas        bypass valve, based on the obtained refrigerant pressure        value(s) and refrigerant temperature value(s), and    -   deriving the actual opening degree of the gas bypass valve based        on the estimated mass flow of gaseous refrigerant through the        gas bypass valve.

According to this embodiment, the mass flow of gaseous refrigerantthrough the gas bypass valve is estimated from measurements ofprevailing refrigerant pressures and/or refrigerant temperatures atselected positions along the refrigerant path. For instance, based onthe pressure prevailing in the receiver and the pressure prevailing inthe suction line, a pressure difference across the gas bypass valve canbe derived. The mass flow of gaseous refrigerant through the gas bypassvalve can be derived from this pressure difference. However, otherrefrigerant pressure values and/or refrigerant temperature values couldbe used for estimating the mass flow of refrigerant in relevant parts ofthe vapour compression system, and this can be used for estimating themass flow of gaseous refrigerant through the gas bypass valve.

Once the estimated mass flow of gaseous refrigerant through the gasbypass valve has been obtained in this manner, the actual opening degreeof the gas bypass valve, which results in this mass flow, can bederived. This could, e.g., be done using a model providingcorrespondence between opening degree of the gas bypass valve and massflow of gaseous refrigerant through the gas bypass valve. The modelcould, e.g., be obtained in an empirical manner during normal operationof the vapour compression system.

The step of deriving an actual opening degree of the gas bypass valvemay be performed based on valve characteristics of the gas bypass valveobtained during normal operation of the gas bypass valve. This may,e.g., be performed in the manner described above.

The step of deriving a target opening degree of the gas bypass valve maybe based on at least an obtained value of a pressure prevailing insidethe receiver. The gas bypass valve is often controlled based on thepressure prevailing in the receiver, and in order to obtain anappropriate pressure level inside the receiver. For instance, in thecase that the pressure prevailing inside the receiver is too high, theopening degree of the gas bypass valve may be increased in order toincrease the mass flow of gaseous refrigerant from the receiver towardsthe compressors, thereby reducing the pressure prevailing inside thereceiver. Similarly, in the case that the pressure prevailing inside thereceiver is too low, the opening degree of the gas bypass valve may bedecreased in order to decrease the mass flow of gaseous refrigerant fromthe receiver towards the compressors, thereby increasing the pressureprevailing inside the receiver. Accordingly, the pressure prevailinginside the receiver provides a suitable indication regarding a targetopening degree for the gas bypass valve, which would match theprevailing operating conditions.

The step of controlling the vapour compression system may be performedin such a manner that a compressor capacity of the compressor(s) is notallowed to decrease below a minimum compressor capacity level. Accordingto this embodiment, it is always ensured that at least one compressor isoperated at least at a minimum compressor speed. In the case that thecompressor capacity of the compressor(s) decreases below a certainminimum level, there is a risk that the vapour compression system isbrought into a state where it is not possible to provide the requiredcooling or heating capacity, and the vapour compression system may stopoperating. It may be necessary to manually restart the vapourcompression system in this case. By ensuring that the compressorcapacity of the compressor(s) is not allowed to decrease below theminimum compressor capacity level, such a situation is avoided.

Alternatively or additionally, the method may further comprise the stepof forcing the compressor(s) to start at predefined time intervals. Thiswill also ensure that the situation described above is avoided.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in further detail with reference tothe accompanying drawings in which

FIG. 1 is a diagrammatic view of a vapour compression system beingcontrolled using a method according to a first embodiment of theinvention,

FIG. 2 is a diagrammatic view of a vapour compression system beingcontrolled using a method according to a second embodiment of theinvention,

FIG. 3 is a diagrammatic view of a vapour compression system beingcontrolled using a method according to a third embodiment of theinvention,

FIG. 4 is a log P-h diagram illustrating a method according to anembodiment of the invention, and

FIG. 5 is a log P-h diagram illustrating a method according to analternative embodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 is a diagrammatic view of a vapour compression system 1 beingcontrolled using a method according to a first embodiment of theinvention. The vapour compression system 1 comprises a compressor unitcomprising a number of compressors 2, two of which are shown, a heatrejecting heat exchanger 3, a high pressure expansion device 4, areceiver 5, an evaporator expansion device 6, in the form of anexpansion valve, an evaporator 7, a gas bypass valve 8 and a suctionline receiver 9 arranged in a refrigerant path.

Refrigerant flowing in the refrigerant path is compressed by thecompressors 2 before being supplied to the heat rejecting heat exchanger3. In the heat rejecting heat exchanger 3, heat exchange takes placewith a secondary fluid flow across the heat rejecting heat exchanger 3in such a manner that heat is rejected from the refrigerant. In the casethat the heat rejecting heat exchanger 3 is in the form of a condenser,the refrigerant passing through the heat rejecting heat exchanger 3 isat least partly condensed. In the case that the heat rejecting heatexchanger 3 is in the form of a gas cooler, the refrigerant passingthrough the heat rejecting heat exchanger 3 is cooled, but it remains ina gaseous state.

The refrigerant leaving the heat rejecting heat exchanger 3 is passedthrough the high pressure expansion device 4 before being supplied tothe receiver 5. The high pressure expansion device 4 may, e.g., be inthe form of a high pressure valve, in the form of an ejector, or in theform of a high pressure valve and an ejector arranged in parallel. Inany event, refrigerant passing through the high pressure expansiondevice 4 undergoes expansion.

In the receiver 5, the refrigerant is separated into a liquid part and agaseous part. The liquid part of the refrigerant is supplied to theevaporator expansion device 6, where it undergoes expansion before beingsupplied to the evaporator 7. In the evaporator 7, heat exchange takesplace with a secondary fluid flow across the evaporator 7 in such amanner that heat is absorbed by the refrigerant, while the refrigerantis at least partly evaporated. The refrigerant leaving the evaporator 7is supplied to the suction line receiver 9, where it is separated into aliquid part and a gaseous part. The gaseous part of the refrigerant issupplied to the compressors 2.

At least part of the gaseous part of the refrigerant in the receiver 5is supplied to the suction line receiver 9, via the gas bypass valve 8.Thus, the refrigerant leaving the evaporator 7 is mixed with gaseousrefrigerant supplied from the receiver 5 in the suction line receiver 9.The mass flow of gaseous refrigerant from the receiver 5 towards thesuction line receiver 9, and thereby towards the compressors 2, is,under normal operating conditions, controlled by controlling an openingdegree of the gas bypass valve 8.

The vapour compression system 1 is further provided with a number ofsensors arranged at selected positions along the refrigerant path. Apressure sensor 10 is arranged near the outlet of the heat rejectingheat exchanger 3 for measuring the pressure of refrigerant leaving theheat rejecting heat exchanger 3. A temperature sensor 11 is arrangednear the outlet of the heat rejecting heat exchanger 3 for measuring thetemperature of refrigerant leaving the heat rejecting heat exchanger 3.A pressure sensor 12 is arranged in the receiver 5 for measuring thepressure prevailing inside the receiver 5. A pressure sensor 13 isarranged near the inlet of the compressors 2 for measuring therefrigerant pressure in the suction line. A temperature sensor 14 isarranged near the inlet of the compressors 2 for measuring therefrigerant temperature in the suction line.

The vapour compression system 1 is controlled on the basis ofmeasurements performed by at least some of the sensors 10, 11, 12, 13,14. For instance, the high pressure expansion device 4 may be controlledon the basis of measurements performed by pressure sensor 10 and/ormeasurements performed by pressure sensor 12. The evaporator expansiondevice 6 may be controlled on the basis of measurements performed bypressure sensor 13 and measurements performed by temperature sensor 14.The gas bypass valve 8 may be controlled on the basis of measurementsperformed by pressure sensor 12 and/or measurements performed bypressure sensor 13.

In the case that the gas bypass valve 8 malfunctions, it is no longerpossible to control the supply of gaseous refrigerant from the receiver5 towards the suction line receiver 9, and thereby towards thecompressors 2, via the gas bypass valve 8. According to the method ofthe invention, continued operation of the vapour compression system 1 isallowed in the following manner.

Initially, an actual opening degree of the gas bypass valve 8 isderived. This could, e.g., include estimating a mass flow of gaseousrefrigerant through the gas bypass valve, based on measurementsperformed by means of one or more of the sensors 10, 11, 12, 13, 14, andsubsequently deriving the actual opening degree of the gas bypass valve8, based on the estimated mass flow.

Furthermore, a target opening degree of the gas bypass valve 8 isderived, based on one or more control parameters of the vapourcompression system 1. Thus, the target opening degree of the gas bypassvalve 8 represents an opening degree which matches the current operatingconditions, and which would be selected if the gas bypass valve 8 wasoperating properly.

Next, the actual opening degree of the gas bypass valve 8 is compared tothe target opening degree of the gas bypass valve 8. Finally, the vapourcompression system 1 is controlled, based on the comparison, and inorder to match a flow of gaseous refrigerant through the gas bypassvalve 8 to the actual opening degree of the gas bypass valve 8. Thus,since the opening degree of the gas bypass valve 8 can not be controlledto provide a mass flow of refrigerant through the gas bypass valve 8which matches the current operating conditions, the operating conditionsare instead adjusted to provide a mass flow of refrigerant through thegas bypass valve 8 which matches the actual opening degree of the gasbypass valve 8.

In particular, in the case that the comparison reveals that the actualopening degree of the gas bypass valve 8 is larger than the targetopening degree of the gas bypass valve 8, the vapour compression system1 is controlled to increase the mass flow of refrigerant through the gasbypass valve 8. This can, e.g., be obtained by decreasing the pressureof refrigerant leaving the heat rejecting heat exchanger 3 and/or byincreasing the temperature of refrigerant leaving the heat rejectingheat exchanger 3.

Similarly, in the case that the comparison reveals that the actualopening degree of the gas bypass valve 8 is smaller than the targetopening degree of the gas bypass valve 8, the vapour compression system1 is controlled to decrease the mass flow of refrigerant through the gasbypass valve 8. This can, e.g., be obtained by increasing the pressureof refrigerant leaving the heat rejecting heat exchanger 3 and/or bydecreasing the temperature of refrigerant leaving the heat rejectingheat exchanger 3.

The pressure of refrigerant leaving the heat rejecting heat exchanger 3can, e.g., be adjusted by adjusting an opening degree of the highpressure expansion device 4 and/or by adjusting a compressor capacity ofthe compressors 2. The temperature of refrigerant leaving the heatrejecting heat exchanger 3 can, e.g., be adjusted by adjusting a fanspeed of a fan driving a secondary fluid flow across the heat rejectingheat exchanger 3.

FIG. 2 is a diagrammatic view of a vapour compression system 1 beingcontrolled using a method according to a second embodiment of theinvention. The vapour compression system 1 is very similar to the vapourcompression system 1 of FIG. 1, and it will therefore not be describedin detail here.

In the vapour compression system 1 of FIG. 2, the high pressureexpansion device is in the form of a high pressure valve 15.Furthermore, the vapour compression system 1 comprises a receivercompressor 16. Gaseous refrigerant is supplied directly from thereceiver 5 to the receiver compressor 16. Accordingly, this gaseousrefrigerant is not subjected to the pressure drop which is introducedwhen the refrigerant passes through the gas bypass valve 8 and is mixedwith the refrigerant leaving the evaporator 9. This reduces the energyrequired in order to compress the refrigerant.

In the case that the gas bypass valve 8 malfunctions, continuedoperation of the vapour compression system 1 can be ensured essentiallyin the manner described above with reference to FIG. 1.

FIG. 3 is a diagrammatic view of a vapour compression system 1 beingcontrolled using a method according to a third embodiment of theinvention. The vapour compression system 1 of FIG. 3 is very similar tothe vapour compression system 1 of FIG. 2, and it will therefore not bedescribed in detail here.

In the vapour compression system 1 of FIG. 3, the high pressureexpansion device is in the form of a high pressure valve 15 and anejector 17 arranged in parallel. Accordingly, some of the refrigerantleaving the heat rejecting heat exchanger 3 passes through the highpressure valve 15, and some of the refrigerant passes through theejector 17 before being supplied to the receiver 5. A secondary inlet 18of the ejector 17 is connected to the suction line. Thereby refrigerantis sucked from the suction line into the ejector 17, reducing the loadon the compressors 2. This even further reduces the energy consumptionof the vapour compression system 1.

In the case that the gas bypass valve 8 malfunctions, continuedoperation of the vapour compression system 1 can be ensured essentiallyin the manner described above with reference to FIG. 1.

FIG. 4 is a log P-h diagram illustrating a method according to anembodiment of the invention. The vapour compression system beingcontrolled could, e.g., be one of the vapour compression systemsillustrated in FIGS. 1-3.

From point 19 to point 20 refrigerant is compressed by the compressors,resulting in an increase in enthalpy and pressure. From point 20 topoint 21 refrigerant passes through the heat rejecting heat exchanger,resulting in a decrease in the temperature of the refrigerant, andthereby a decrease in enthalpy, while the pressure remains substantiallyconstant. From point 21 to point 22 the refrigerant passes through thehigh pressure expansion device, resulting in a decrease in pressure,while the enthalpy remains substantially constant. From point 23 topoint 24 the liquid part of the refrigerant passes through theevaporator expansion device, also resulting in a decrease in pressure,while the enthalpy remains substantially constant. From point 24 topoint 25 the refrigerant passes through the evaporator, resulting in anincrease in the temperature of the refrigerant, and thereby an increasein enthalpy, while the pressure remains substantially constant. Frompoint 26 to point 25 the gaseous part of the refrigerant in the receiverpasses through the gas bypass valve, resulting in a decrease inpressure, while the enthalpy remains substantially constant.

In FIG. 4, three different paths, corresponding to three differentpressure values of the refrigerant leaving the heat rejecting heatexchanger are illustrated. In the three paths, the temperature ofrefrigerant leaving the heat rejecting heat exchanger is the same,illustrated by isotherm 27. A first path, point 19-point 20 a-point 21a-point 22 a, corresponds to a low pressure value. A second path, point19-point 20 b-point 21 b-point 22 b, corresponds to a medium pressurevalue. A third path, point 19-point 20 c-point 21 c-point 22 c,corresponds to a high pressure level. The position of point 22 along theenthalpy axis reflects the gas to liquid ratio in the receiver. Theposition of point 22 a illustrates a situation in which the gas toliquid ratio is high, the position of point 22 b illustrates a situationin which the gas to liquid ratio is medium, and the position of point 22c illustrates a situation in which the gas to liquid ratio is low. Thegas to liquid ratio in the receiver affects the mass flow of gaseousrefrigerant through the gas bypass valve.

Accordingly, by adjusting the pressure of the refrigerant leaving theheat rejecting heat exchanger, the gas to liquid ratio in the receivercan be adjusted, thereby adjusting the mass flow of gaseous refrigerantthrough the gas bypass valve. More particularly, increasing the pressureof refrigerant leaving the heat rejecting heat exchanger results in adecrease in the gas to liquid ratio of refrigerant in the receiver, andthereby in a decreased mass flow of gaseous refrigerant through the gasbypass valve. Similarly, decreasing the pressure of refrigerant leavingthe heat rejecting heat exchanger results in an increase in the gas toliquid ratio of refrigerant in the receiver, and thereby in an increasedmass flow of gaseous refrigerant through the gas bypass valve.

FIG. 5 is a log P-h diagram illustrating a method according to analternative embodiment of the invention. The log P-h diagram of FIG. 5is similar to the log P-h diagram of FIG. 4, and it will therefore notbe described in detail here.

In FIG. 5, three different paths, corresponding to three differenttemperature values of the refrigerant leaving the heat rejecting heatexchanger, illustrated by three isotherms 27 d, 27 e, 27 f, areillustrated. In the three paths, the pressure of refrigerant leaving theheat rejecting heat exchanger is the same. A first path, point 19-point20-point 21 d-point 22 d, corresponds to a high temperature value. Asecond path, point 19-point 20-point 21 e-point 22 e, corresponds to amedium temperature value. A third path, point 19-point 20-point 21f-point 22 f, corresponds to a low temperature value. As describedabove, the position of point 22 along the enthalpy axis reflects the gasto liquid ratio in the receiver, which affects the mass flow of gaseousrefrigerant through the gas bypass valve.

Accordingly, by adjusting the temperature of the refrigerant leaving theheat rejecting heat exchanger, the gas to liquid ratio in the receivercan be adjusted, thereby adjusting the mass flow of gaseous refrigerantthrough the gas bypass valve. More particularly, increasing thetemperature of refrigerant leaving the heat rejecting heat exchangerresults in an increase in the gas to liquid ratio of refrigerant in thereceiver, and thereby in an increased mass flow of gaseous refrigerantthrough the gas bypass valve. Similarly, decreasing the temperature ofrefrigerant leaving the heat rejecting heat exchanger results in adecrease in the gas to liquid ratio of refrigerant in the receiver, andthereby to a decreased mass flow of gaseous refrigerant through the gasbypass valve.

While the present disclosure has been illustrated and described withrespect to a particular embodiment thereof, it should be appreciated bythose of ordinary skill in the art that various modifications to thisdisclosure may be made without departing from the spirit and scope ofthe present disclosure.

What is claimed is:
 1. A method for controlling a vapour compressionsystem, the vapour compression system comprising at least onecompressor, a heat rejecting heat exchanger, a high pressure expansiondevice, a receiver, an evaporator expansion device, an evaporator and agas bypass valve, arranged in a refrigerant path, the method comprisingthe steps of: registering that the gas bypass valve is malfunctioning,deriving an actual opening degree of the gas bypass valve, deriving atarget opening degree of the gas bypass valve, based on one or morecontrol parameters of the vapour compression system, comparing theactual opening degree of the gas bypass valve to the target openingdegree of the gas bypass valve, and controlling the vapour compressionsystem based on the comparison, and in order to match a mass flow ofgaseous refrigerant through the gas bypass valve to the actual openingdegree of the gas bypass valve.
 2. The method according to claim 1,wherein the step of controlling the vapour compression system based onthe comparison comprises the steps of: in the case that the comparisonreveals that the actual opening degree of the gas bypass valve is largerthan the target opening degree of the gas bypass valve, controlling thevapour compression system to increase the mass flow of gaseousrefrigerant through the gas bypass valve, and in the case that thecomparison reveals that the actual opening degree of the gas bypassvalve is smaller than the target opening degree of the gas bypass valve,controlling the vapour compression system to decrease the mass flow ofgaseous refrigerant through the gas bypass valve.
 3. The methodaccording to claim 2, wherein the step of increasing the mass flow ofgaseous refrigerant through the gas bypass valve comprises decreasing apressure of refrigerant leaving the heat rejecting heat exchanger and/orincreasing a temperature of refrigerant leaving the heat rejecting heatexchanger.
 4. The method according to claim 2, wherein the step ofdecreasing the mass flow of gaseous refrigerant through the gas bypassvalve comprises increasing a pressure of refrigerant leaving the heatrejecting heat exchanger and/or decreasing a temperature of refrigerantleaving the heat rejecting heat exchanger.
 5. The method according toclaim 1, wherein the step of controlling the vapour compression systembased on the comparison step comprises adjusting an opening degree ofthe high pressure expansion device, adjusting a secondary fluid flowacross the heat rejecting heat exchanger and/or adjusting a compressorcapacity of the compressor(s).
 6. The method according to claim 1,wherein the step of deriving an actual opening degree of the gas bypassvalve comprises the steps of: obtaining one or more refrigerant pressurevalues and one or more refrigerant temperature values at selectedpositions along the refrigerant path, estimating a mass flow of gaseousrefrigerant through the gas bypass valve, based on the obtainedrefrigerant pressure value(s) and refrigerant temperature value(s), andderiving the actual opening degree of the gas bypass valve based on theestimated mass flow of gaseous refrigerant through the gas bypass valve.7. The method according to claim 1, wherein the step of deriving anactual opening degree of the gas bypass valve is performed based onvalve characteristics of the gas bypass valve obtained during normaloperation of the gas bypass valve.
 8. The method according to claim 1,wherein the step of deriving a target opening degree of the gas bypassvalve is based on at least an obtained value of a pressure prevailinginside the receiver.
 9. The method according to claim 1, wherein thestep of controlling the vapour compression system is performed in such amanner that a compressor capacity of the compressor(s) is not allowed todecrease below a minimum compressor capacity level.
 10. The methodaccording to claim 1, further comprising the step of forcing thecompressor(s) to start at predefined time intervals.
 11. The methodaccording to claim 3, wherein the step of decreasing the mass flow ofgaseous refrigerant through the gas bypass valve comprises increasing apressure of refrigerant leaving the heat rejecting heat exchanger and/ordecreasing a temperature of refrigerant leaving the heat rejecting heatexchanger.
 12. The method according to claim 2, wherein the step ofcontrolling the vapour compression system based on the comparison stepcomprises adjusting an opening degree of the high pressure expansiondevice, adjusting a secondary fluid flow across the heat rejecting heatexchanger and/or adjusting a compressor capacity of the compressor(s).13. The method according to claim 3, wherein the step of controlling thevapour compression system based on the comparison step comprisesadjusting an opening degree of the high pressure expansion device,adjusting a secondary fluid flow across the heat rejecting heatexchanger and/or adjusting a compressor capacity of the compressor(s).14. The method according to claim 4, wherein the step of controlling thevapour compression system based on the comparison step comprisesadjusting an opening degree of the high pressure expansion device,adjusting a secondary fluid flow across the heat rejecting heatexchanger and/or adjusting a compressor capacity of the compressor(s).15. The method according to claim 2, wherein the step of deriving anactual opening degree of the gas bypass valve comprises the steps of:obtaining one or more refrigerant pressure values and one or morerefrigerant temperature values at selected positions along therefrigerant path, estimating a mass flow of gaseous refrigerant throughthe gas bypass valve, based on the obtained refrigerant pressurevalue(s) and refrigerant temperature value(s), and deriving the actualopening degree of the gas bypass valve based on the estimated mass flowof gaseous refrigerant through the gas bypass valve.
 16. The methodaccording to claim 3, wherein the step of deriving an actual openingdegree of the gas bypass valve comprises the steps of: obtaining one ormore refrigerant pressure values and one or more refrigerant temperaturevalues at selected positions along the refrigerant path, estimating amass flow of gaseous refrigerant through the gas bypass valve, based onthe obtained refrigerant pressure value(s) and refrigerant temperaturevalue(s), and deriving the actual opening degree of the gas bypass valvebased on the estimated mass flow of gaseous refrigerant through the gasbypass valve.
 17. The method according to claim 4, wherein the step ofderiving an actual opening degree of the gas bypass valve comprises thesteps of: obtaining one or more refrigerant pressure values and one ormore refrigerant temperature values at selected positions along therefrigerant path, estimating a mass flow of gaseous refrigerant throughthe gas bypass valve, based on the obtained refrigerant pressurevalue(s) and refrigerant temperature value(s), and deriving the actualopening degree of the gas bypass valve based on the estimated mass flowof gaseous refrigerant through the gas bypass valve.
 18. The methodaccording to claim 5, wherein the step of deriving an actual openingdegree of the gas bypass valve comprises the steps of: obtaining one ormore refrigerant pressure values and one or more refrigerant temperaturevalues at selected positions along the refrigerant path, estimating amass flow of gaseous refrigerant through the gas bypass valve, based onthe obtained refrigerant pressure value(s) and refrigerant temperaturevalue(s), and deriving the actual opening degree of the gas bypass valvebased on the estimated mass flow of gaseous refrigerant through the gasbypass valve.
 19. The method according to claim 2, wherein the step ofderiving an actual opening degree of the gas bypass valve is performedbased on valve characteristics of the gas bypass valve obtained duringnormal operation of the gas bypass valve.
 20. The method according toclaim 3, wherein the step of deriving an actual opening degree of thegas bypass valve is performed based on valve characteristics of the gasbypass valve obtained during normal operation of the gas bypass valve.